Gypsum fiberboard is a construction material made from admixing water, stucco and cellulosic fibers to form a wet mixture, and permitting the stucco, also known as gypsum hemihydrate, to cure to form a set gypsum dihydrate-containing board. Unlike paper-faced wallboard, which is typically a laminar construction including a relatively weak gypsum core disposed between two relatively heavy paper sheets, fiberboard is typically unfaced. It is generally known that wallboard relies upon these paper facings to provide as much as 90% of the requisite bending strength, whereas fiberboard relies upon an intimate mixture of gypsum dihydrate crystals and cellulosic fibers which adhere together to distribute applied forces uniformly throughout the composite structure. This unique feature of fiberboard has made it attractive in applications requiring a high degree of mechanical strength, such as in fire door cores and edge banding.
Early fiberboard manufacturing processes, such as the one disclosed in Porter et al., U.S. Pat. No. 2,076,349, taught the mixing of calcined gypsum hemihydrate, paper fibers, and "excess water" (over and above that required to fully hydrate the hemihydrate gypsum) together to form a slurry. The slurry was placed into a mold and then subjected to a pressure of up to about 1,000 psi so that most of the excess water could be squeezed from the mixture. The resulting "green board", i.e. not fully set, contained about 30-35 wt. % moisture which was later removed by drying in a kiln oven. Fiberboards produced by this process were strong, having modulus of rupture values approaching as high as 1,750 lbs. per square inch, but low efficiency and the costs associated with removing all that water made the boards too expensive. Porter suggested a continuous operation for producing endless webs of fiberboard, which could have helped to alleviate these costs, but his disclosure failed to provide sufficient details for practicing such an operation.
In some of the more modern processes, such as in the method described in Take et al., U.S. Pat. No. 4,645,548, sheet making equipment has been employed to promote more continuous manufacturing of fiberboard. Moreover, readily available and inexpensive forms of gypsum dihydrate, such as flue gas desulfurization and phosphoric acid dihydrate industrial by-products have been employed as a filler in these boards to further reduce costs. Take discloses forming a slurry with a mixture of hydraulic gypsum, gypsum dihydrate, organic and inorganic fibers, a setting retarder, and water. The slurry is taken up on a sheet making roll, which is partially immersed in the slurry bath, and which transfers the slurry to a passing belt. The resulting green sheet is then cut to length, laminated to another board and press-molded. Although this reference teaches that as much as 50 wt. % gypsum dihydrate could be added to the fiberboard initial slurry without significantly affecting board strength, Take's method fails to take full advantage of the bonding properties of gypsum, which are largely due to the crystallization of the hemihydrate into the dihydrate form. Thus, the use of dihydrate as a filler has not been particularly popular.
Recent developments in Germany, such as those disclosed in Kraemer et al., "Gypsum Fibre Boards for the Dry Interior Finish Construction", Holz-Zentralblatt, Stuttgart, Vol. 111, No. 11 (January, 1985), suggest that fiberboard can be produced in a continuous "dry process". The dry process, commercialized by G. Siempelkamp GmbH & Co., Krefeld, Germany, begins with a dry mixture of plaster, gypsum, and paper fibers which is thoroughly blended in a high-speed continuous flow-mixer. The mixture is conveyed to a bunker of a matformer, where it is then formed into an endless mat of a dry plaster-gypsum-fiber mix on a spreading belt. The endlessly formed mat is then transferred onto a screen belt and wetted with a minimum amount of water. Vacuum boxes located beneath the wetting unit facilitate the penetration of water through the cross section of the mat. The wetted mat then enters a movable, open cycle press, where it is pressed between a plastic coated texture belt running synchronously on top of the mat. The water squeezed out from the mat is drained into a press pit. After the expiration of the pressing time, the press opens and returns into its initial starting position. The pressed mat is then ready for cutting and subsequent setting and curing operations.
Although the dry process is now used extensively in Europe, comparative testing of gypsum fiberboards produced with the dry and wet processes has demonstrated that boards produced from a slurry containing water over and above that required to hydrate the hemihydrate are more homogeneous in appearance and are about 70% stronger in flexural strength tests than comparable thickness, dry process boards.
Other manufacturers, such as Vogt, U.S. Pat. No. 4,840,688, have sought to combine the benefits of using cheap industrial dihydrate waste as a starting material and the uniformity and strength provided by a wet process in a single manufacturing line. Vogt teaches the wet shaping of gypsum dihydrate and wet-digested fibers, followed by the removal of water, the dry recrystallization of the dihydrate to hemihydrate by heating at atmospheric pressure, and then the subsequent conversion back to the dihydrate by the addition of water. Despite his aggressive attempt at using multiple recrystallizations of gypsum to maximize strength, the complexity and costs associated with Vogt's process detracts from its commercial value.
Accordingly, there is a need for continuous processing equipment for manufacturing gypsum fiberboards having a high degree of uniformity and great strength. This equipment should also be easy to implement and not require complicated elements which would be difficult to maintain and run.
Applicants have recognized an additional and significant disadvantage of prior art equipment, apparatus, and processes. This disadvantage stems from the need in prior processes to purchase expensive equipment to produce fiberboard products and the need to purchase equally expensive equipment to produce wallboard products. Furthermore, because the size of the processing units required to produce such products are physically large, the capital demands associated with putting both a wallboard and fiberboard manufacturing facility on-line according to prior processes severely hampered the commercial viability of such prior equipment and processes. Applicants have discovered a commercially viable alternative which overcomes this substantial deficiency in prior processes and equipment.